NOTE: This is a condensed excerpt from my forthcoming (2014) commentary on Philippians.
Paul continues his description of those who preach Christ because of their delight in him by adding that they do it [i.e. preach Christ] out of love. The fact that Paul does not further specify the object of this love suggests that the term should be understood in a very broad manner. He could be speaking of God’s love for his people and the world as the driving force behind preaching the gospel. In 2 Cor 5:14 describes his own motivation for ministry in terms of Christ’s love compelling him to action. Or Paul could have in mind the believer’s love for God. In Philemon 1:4-5 Paul expresses his gratitude to God “because I hear of your love and of the faith that you have toward the Lord Jesus and for all the saints.” The overlap between the believer’s love for Christ and love for his people fits Philippians 1:16 nicely as well. Indeed, Paul can even claim that the entire Law is fulfilled in living out the command to love your neighbor as you love yourself (Gal 5:13-14). Finally, Paul could also have in mind their love for the gospel itself. Although Paul never explicitly speaks of loving the gospel, it is apparent from his life and letters that he loved the gospel because it revealed the good news of Jesus Christ and his redemption. Such love for Christ and the gospel led him to tireless labor in extremely difficult circumstances (cp. 1 Cor 4:9-13; 1 Thes 2:8-9). Because Paul has left the term so open-ended here in Phil 1:16, we would be wise not to specify too narrowly the kind of love Paul has in view. Those who truly love God and love others will share the gospel.
 Rather than use the preposition dia plus the accusative to indicate motive as he did in 1:15, here Paul uses the preposition ek plus the genitive. There is no appreciable difference in meaning.